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Coal and biomass cofiring: CFD modeling 109

(Centeno et al., 2015). Besides the E-EWBM, some other models, such as the spectral
lineebased WSGGM and the full-spectrum k-distribution method (Modest, 2003),
are also practically accurate, computationally competitive, and able to address the
practical limitations of the WSGGMs. These models are more straightforward in
nongray calculation than the E-EWBM is. As a result, they can also be reliably
used in general combustion CFD.
In solid fuel particles, the particles also have strong impacts on radiative heat trans-
fer, as seen in Eqs. (4.2), (4.4) and (4.5). In suspension-fired furnaces, in which solid
particle concentrations are high, particle radiation can overwhelm gas radiation even
under oxy-fuel combustion conditions (Yin, 2015, 2016). As a result, it is important
to derive reliable composition-dependent models for particle radiative properties.
For instance, instead of the constant particle emissivity and the constant particle scat-
tering factor as commonly used in solid fuel combustion CFD, a conversion degree-
dependent particle emissivity and scattering factor can be used as follows (Yin, 2015),

(
ε p ¼ 0:4$U C þ 0:6
(4.19)

f p ¼ 0:9 U VM;C þ 0:6 1 U VM;C
where U C and U VM,C represents the fraction of unburned char and the fraction of
unburned combustibles (i.e., volatile matters and char) in a fuel particle, respectively.
In effect, the particle emissivity ε p varies from 1.0 for unburned coal to 0.6 for residual
ash. The particle scattering factor changes from 0.9 for unburned coal (yielding a lower
particle scattering coefficient) to 0.6 for residual ash particles (corresponding to a
higher particle scattering coefficient), according to Eq. (4.5).

4.6.2 Modeling of combustion chemistry under oxy-fuel
conditions

The high-concentration CO 2 in oxy-fuel flames also has important chemical effects,
via homogeneous and/or heterogeneous reactions, and yields higher CO concentra-
tions (Toftegaard et al., 2010; Chen et al., 2012; Yin and Yan, 2016). H 2 O vapor in
oxy-fuel combustion also has chemical effects: it can promote or inhibit CO oxidation
depending on the specific conditions. Global combustion mechanisms such as the 2-
step and 4-step mechanisms presented in section 3.5 are commonly used in combustion
CFD due to their good computational efficiency. However, none of them has been vali-
dated against oxy-fuel experimental data.
Andersen et al. (2009) refined the 2-step and 4-step global mechanisms for oxy-fuel
combustion by using a detailed chemical kinetic mechanism as the reference model. In
the refined schemes, the initiating reactions involving hydrocarbon and oxygen are
retained, whereas the H 2 eCOeCO 2 reactions are modified to improve prediction of
the major species concentrations. A comparative CFD analysis of a propane oxy-
fuel flame is also made. Compared with the original versions, the refined WD 2-step
mechanism improves the prediction of the temperature field and CO in the postflame
zone, and the refined JL 4-step mechanism slightly better predicts the CO profile in the